...or put your two crontab entries on one line: Use unix shell boolean "and" to force the shell to run multiple commands one by one...

As I read the problem, this does not make any sense. My understanding is that this is not an issue of multiple commands, but rather a single command which is run frequently enough that it can sometimes overlap itself. Like, say, a cronjob run once per hour that occasionally take 70 minutes to complete, so that cron is trying to start a new instance before the old instance is finished (which can be quite nasty).

My personal preference in cases like this is to put a routine at the beginning of the script to check running processes for another process of the same name.
...or to change cron to run the task infrequently enough that it doesn't overlap. :-)